Two different classes of image scanners are known in the art. Flatbed scanners and drum scanners.
In a flatbed scanner, the original to be scanned has to be held extremely flat in order for all areas of an image on the original to be positioned in focus of the optics of the image scanner during recording of that area. For this purpose, the original is typically placed in a holder comprising two holder parts with glass plates. The original may be clamped between the two glass plates.
It is a disadvantage of known holders for flatbed scanners that the manual attachment of the original in the holder does not assure complete contact between the original and the glass plates so that Newton rings may be created by the partial separation between these surfaces.
It is a another disadvantage that the holder requires regular and careful cleaning to avoid dust and dirt to accumulate on the glass surfaces. Dirty surfaces impair the quality of scanned images and may scratch originals.
It is yet another disadvantage that the holder requires careful handling to keep the glass plates free of scratches.
It is still another disadvantage that the accurate positioning of the original in the holder require skills of the operator and is tedious and time consuming.
It is a further disadvantage that the holder can not apply sufficient strong forces to the original to stretch it to the required planar form. Typically, the original forms concave and convex buckles between the glass plates of the holder.
In a drum scanner, the original to be scanned is mounted on a transparent image reading drum. The original is manually attached and fixed onto the cylindrical surface of the image reading drum with adhesive tape, and is detached from the drum by peeling the adhesive tape off the original.
It is a disadvantage of the drum scanner that the attachment of the original to the drum does not assure complete contact of the original with the image reading drum, and in many cases, its poor contact with or partial separation from the drum causes Newton's rings.
It is another disadvantage of the drum scanner that because of the high rotational speed of the drum, the original has to be firmly attached to the drum to withstand high centrifugal forces. The firm mounting of the original requires skill and is time consuming.